U.S. Pat. No. 4,533,372 to Jaime A. Valencia and Robert D. Denton discloses a method and apparatus for separating carbon dioxide and other acid gases from methane by treating the feedstream in a controlled freezing zone (hereinafter "Controlled Freezing Zone Process"). The Controlled Freezing Zone Process is one which utilizes a controlled freezing zone, which permits the solidification of carbon dioxide in a controlled manner, and which simultaneously allows the thermodynamic distillation of a feedstream mixture containing carbon dioxide and methane in one distillation column.
Prior to the Controlled Freezing Zone Process, complexities in the thermodynamics of carbon dioxide-methane mixtures made difficult, if not impossible, the separation of such mixtures via conventional cryogenic distillation. These complexities relate to the formation of solid carbon dioxide at equilibrium with the vapor-liquid mixtures of carbon dioxide in methane at the particular conditions of temperature, pressure, and composition at which the cryogenic distillation of these compounds take place. The formation of solids in a distillation tower has the undesirable potential effect of plugging the tower and its associated equipment.
The Controlled Freezing Zone Process is a method for separating carbon dioxide and other acid gases from methane using cryogenic distillation in combination with a controlled freezing zone. Specifically, the invention includes a cryogenic distillation tower having a zone which is adapted to handling the production of solid carbon dioxide. The zone is designed to allow the formation of carbon dioxide solids and to allow contact of vapor, liquid, and solid necessary for separation by distillation to occur.
Optimum operation of the Controlled Freezing Zone Process requires the liquid sprayed into the freezing zone of the tower ("freezing zone liquid feedstream") to be at conditions which are close to but not quite at carbon dioxide solidification conditions. It is highly undesirable for the freezing zone liquid feedstream to attain solidification conditions before entering the freezing zone.
In a well balanced system, the solidification of carbon dioxide will occur only in the freezing zone and not in any other part of the system, particularly in the freezing zone liquid feedstream lines. A heat leak into the lines containing the freezing zone liquid feedstream on its way to the freezing zone can lead to the solidification of carbon dioxide prior to its introduction into the freezing zone. For example, such heat leaks can occur due to insufficient or defective piping insulation, or by dissipation of the heat generated by the freezing zone liquid feedstream pumps. Solidification conditions may also occur for other reasons, such as for example, a drop in pressure on the suction side of the spray pump which sprays the liquid into the freezing zone.
There is a need therefore for a simple method and apparatus for optimizing and controlling the solidification conditions of carbon dioxide in the freezing zone liquid feedstream lines. Preferably such method and apparatus will maintain the freezing zone liquid feedstream at conditions close to, but not quite at, solidification conditions. In addition, such method and apparatus will allow optimum operation of the controlled freezing zone in the presence of changing conditions which may occur in the freezing zone liquid feedstream lines.